Networks enable computers and other devices to communicate. For example, networks can carry data representing video, audio, e-mail, and so forth. Typically, data sent across a network is divided into smaller messages known as packets. By analogy, a packet is much like an envelope you drop in a mailbox. A packet typically includes “payload” and a “header”. The packet's “payload” is analogous to the letter inside the envelope. The packet's “header” is much like the information written on the envelope itself. The header can include information to help network devices handle the packet appropriately. For example, the header can include an address that identifies the packet's destination. A given packet may “hop” across many different intermediate network devices (e.g., “routers”; “bridges” and “switches”) before reaching its destination.
A number of network protocols cooperate to handle the complexity of network communication. For example, a protocol known as Transmission Control Protocol (TCP) provides “connection” services that enable remote applications to communicate. That is, much like picking up a telephone and assuming the phone company will make everything in-between work, TCP provides applications on different computers with simple commands for establishing a connection (e.g., CONNECT and CLOSE) and transferring data (e.g., SEND and RECEIVE). Behind the scenes, TCP transparently handles a variety of communication issues such as data retransmission, adapting to network traffic congestion, and so forth.
To provide these services, TCP operates on packets known as segments. Generally, a TCP segment travels across a network within (“encapsulated” by) a larger packet such as an Internet Protocol (IP) datagram. The payload of a segment carries a portion of a stream of data sent across a network. A receiver can reassemble the original stream of data from the received segments.
Potentially, segments may not arrive at their destination in their proper order, if at all. For example, different segments may travel very different paths across a network. Thus, TCP assigns a sequence number to each data byte transmitted. This enables a receiver to reassemble the bytes in the correct order. Additionally, since every byte is sequenced, each byte can be acknowledged to confirm successful transmission.
Occasionally, data transmission errors may occur. For example, due to signal noise, a “1” bit within a segment may be accidentally changed to a “0” or vice-versa. To enable detection of errors, the TCP header includes a “checksum” field. The value of the checksum is computed by storing zeroes in the segment's checksum field and then summing each byte in the segment using an arithmetic operation known as “one's complement addition”.